The fluidity of a powder processing bed is an important parameter for quality control. There is strong economic incentive to measure the fluidity during processing. It allows time for any remedial control action, thereby preventing the formation of an off-specification product at the end of the run.
The fluidity of a phosphor fluid bed depends on the inherent cohesiveness of the material, the fluidizing gas flow rate, and the spatial distribution of gas in the bed volume. The cohesiveness of phosphors is well documented, and researchers have found various additives which, when blended with the phosphor in optimum amounts, reduce the inherent cohesiveness of the phosphor. Techniques of determining bed fluidity, however, have deficiencies. Current methods rely on visual observation of the bed, measurements of bed expansion, elutriation loss, and normalized bed pressure drop.
While visual observation of the bed is probably the best nonquantitative method of estimating the bed fluidity, it requires a transparent wall material for the fluid bed. This restricts the wall to materials like quartz and high temperature glasses which can stand the elevated temperatures needed for gas/solid reactions in the bed. Since these materials have the severe handicap of being brittle, a major safety hazard is introduced into industrial processes, especially when the chemicals being processed in the fluid bed are pyrophoric. An example of such a chemical is trimethyl aluminum whose use is discussed in U.S. Pat. No. 4,678,970.
It should be noted that bed expansion by itself is not a complete and definite measure of bed fluidity. In fluidization of cohesive powders, like phosphors for example, the bed may expand simply due to the presence of multiple cracks in the bed, without displaying significant powder movement. Bed expansion measurements for estimation of bed fluidity should be complemented by measurements of elutriation loss and/or normalized bed pressure drop.
One can physically measure the bed expansion using a scale on the external wall of the fluid bed. This would require a transparent wall, with its associated safety disadvantage. One can also measure bed expansion using ultrasonic sensors located in the freeboard. The accuracy of these units is often questionable due to interference of the signal by the presence of a powder dust cloud in the freeboard. X-ray bed level detection systems are sometimes used, but several industries prefer not to adopt radiation methods for health reasons.
Other parameters remaining the same, a higher elutriation loss results from a more mobile fluid bed system than from one where the powder movement is slight. Measurements of elutriation loss involve weighing the mass of powder lost from the bed over a certain period of time. While this is feasible in a laboratory process, industrial processes in which flammable and/or pyrophoric chemicals are being used are less suited for such measurements. It can generally be said that a treatment which increases both the bed expansion and the elutriation loss makes the bed more fluid. This means that if several different concentrations of an additive were being tested for their effect on bed fluidity, that treatment which gave the highest bed expansion and the highest elutriation loss could be considered as producing the most mobile bed. Elutriation losses are not desired, however, in industrial processing, especially when expensive powders are being handled. It is possible to minimize bed material loss, without sacrificing bed expansion, by suitable design of the freeboard section.
In a gas fluidized bed with no channeling, almost all of the bed weight is supported by the pressure drop of the gas. This is typically the case in gas fluidization of Geldart type B and A materials. As one moves to finer materials, however, gas channeling starts, bubbles disappear and are largely replaced by a network of vertical and inclined cracks. Under these conditions the ratio of bed pressure drop to bed material weight, often referred to as the normalized bed pressure drop, is less than unity. Bed fluidity decreases as the normalized bed pressure drop deviates more from unity. The bed mass used in the calculation refers to the initial mass of material charged to the bed.
Direct measurement of bed pressure drop is easily accomplished for Geldart type A and B materials, by installing one or more pressure transducers just above the distributor plate. A similar procedure is problematic for Geldart Class C materials (like phosphors) because any screen like device used to isolate the sensor from the bed material is easily clogged by the fine particles. For these materials, the bed pressure drop is usually found by installing a pressure transducer in the plenum section of the fluid bed. This provides the total pressure drop during actual operation, and the distributor pressure drop when gas is passed through the plate with no bed present. Subtraction of the latter data from the former yields the bed pressure drop. It is possible to develop a computer based system to calculate in real time the bed pressure drop and the normalized bed pressure drop.
A problem with this method of determining bed pressure drop occurs when one or more of the precursor chemicals used in the CVD reactions in the bed is prone to pyrolysis. Pyrolysis, or thermal decomposition in the absence of oxygen, at the distributor plate can lead to partial plugging of the pores of the plate. The distributor plate pressure drop is an increasing function of the gas flow rate per unit area of plate. As the plate gradually plugs up, the flow rate per unit area increases because the flow rate of the gases is maintained essentially constant by the mass flow controllers. The resulting upward shift in the distributor characteristic, pressure drop versus gas flow rate, will result in an error in the computation of the bed pressure drop by the subtraction procedure. The error will be an over estimate of the bed pressure drop, resulting in a rosier picture that shows a lesser extent of channeling than that which really exists.
Heretofore, prior art techniques for monitoring the fluidity of a fluidized bed have been deficient.